1. Technical Field
The present invention relates to a burner for manufacturing a porous glass base material and a manufacturing apparatus for manufacturing a porous glass base material for optical fiber, in which, according to the optical fiber base material manufacturing method, a porous glass base material is obtained that has a desired density distribution and the yield of the base material is improved.
2. Related Art
A variety of conventional methods have been proposed for manufacturing optical fiber base material. Among these methods, VAD is a commonly known method that includes affixing a starting member to a shaft that rises while rotating, lowering the starting member into a reaction chamber, and using a core deposition burner and cladding deposition burner arranged within the reaction chamber at prescribed angles relative to the axial direction of the starting member to generate and deposit glass fine particles on the tip of the starting member, thereby manufacturing a porous glass base material from a core layer and a cladding layer. With this method, a high production rate cannot be expected, but it is the preferable method for obtaining a desired refractive index distribution.
FIG. 1 is a schematic view of a manufacturing apparatus for manufacturing a porous glass base material for optical fiber using VAD. The reaction container is formed of a deposition chamber 2, including a gas inlet 4 and an exhaust outlet 5, and a storage chamber 3 that stores the manufactured object, and a plurality of burners are used to synthesize the porous glass base material 1. The starting member is inserted into the deposition chamber 2, and as the starting member is being raised and rotated, the reaction gas is supplied to each burner, hydrolysis occurs in the oxyhydrogen flame, and the synthesized glass fine particles are deposited on the starting member, thereby manufacturing the porous glass base material 1. The burners used here are usually quartz glass burners, and there are a plurality of burners including a core deposition burner 6 arranged pointing at the tip of the starting member and a first cladding deposition burner 7 and second cladding deposition burner 8 that are arranged pointing at the side surface of the starting member. These burners are each arranged at a prescribed angle relative to the axis on which the starting member is pulled up. The manufactured porous glass base material 1 undergoes transparent vitrification in an electric furnace to become a preform for optical fiber.
A porous glass base material can also be manufactured using OVD. FIG. 2 is a schematic view of a manufacturing apparatus for manufacturing a porous glass base material for optical fiber using OVD. The starting member is formed by fusing the ends of a core rod 14 to dummy rods 15, and the core rod 14 is attached to a rotating chuck 17 via one of the dummy rods 15, to be supported in a manner to be rotatable on the axis thereof. Burners 16 are arranged in series in a manner to be movable left and right, and are pointed at the starting member. Combustible gas, combustion assisting gas, and inert gas are blown along with glass raw material gas (SiCl4) from the burners 16, glass fine particles are synthesized by a hydrolytic reaction in an oxyhydrogen flame, and these glass fine particles are deposited on the starting member, thereby obtaining the porous glass base material 18. Furthermore, a manufacturing apparatus for manufacturing a porous glass base material for optical fiber that uses OVD further includes an exhaust hood 19. With this OVD method, a plurality of burners are lined up in series along the starting member that rotates and is arranged horizontally, and the porous glass base material 18 is manufactured by moving the burners or the starting member relative to each other such that the glass fine particles generated in the flames of the burners are deposited. This method provides a high production rate. The porous glass base material 18 obtained in this manner is passed through a heating furnace formed of a heat resistant material to undergo transparent vitrification, thereby obtaining the optical fiber base material.
In these conventional manufacturing methods, concentric multi-tube burners can be used as the burners for synthesizing the glass fine particles, but burners having this structure cannot sufficiently mix the glass raw material gas, the combustible gas, and the combustion assisting gas, and therefore there is not sufficient generation of the glass fine particles. As a result, the yield cannot be improved and high-speed synthesis is difficult. In order to solve this problem, Japanese Patent No. 1,773,359 proposes a multi-nozzle burner in which small-diameter combustion assisting gas emission nozzles are arranged in a manner to surround a central raw material gas nozzle in a combustible gas emission nozzle.
In recent years, base material has become larger in an attempt to decrease cost, and this has caused the following problems. With the VAD method shown in FIG. 1, the first cladding deposition burner 7 arranged above the core deposition burner 6 is pointed diagonally upward. Therefore, in the deposition region, as shown in FIG. 3, the deposition density of the bottom side 10 of the flame tends to be high while the deposition density of the top side 11 of the flame tends to be low. The burner central axis 9 is the line through the central axis of the first cladding deposition burner 7.
Here, when the amount of combustible gas supplied to the first cladding deposition burner 7 is increased in order to achieve a suitable density in the top side 11 of the flame, the density of the bottom side 10 of the flame becomes too high and air bubbles remain during the vitrification. On the other hand, when the amount of combustible gas is reduced in order to achieve a suitable density in the bottom side 10 of the flame, the density in the top side 11 of the flame becomes too low and cracking occurs during deposition.
When the OVD method is used, the glass fine particles are deposited on the core rod 14 using an apparatus such as shown in FIG. 2, but as shown in FIGS. 4 and 5, the inter-burner regions 20 lined up in series are strongly heated from both sides by the flames of adjacent burners, and therefore the deposition density in these portions tends to be locally increased. As a result, when the amount of combustible gas supplied to the burners is increased in order to realize a suitable overall density for the glass base material 18, the deposition density of the inter-burner regions 20 become locally too high and air bubbles remain during the vitrification. On the other hand, when the amount of combustible gas is reduced in order to realize a suitable deposition density in portions of the inter-burner regions 20, the overall density of the porous glass base material 18 decreases and cracking occurs during deposition.